1. Field of the Invention
The present invention relates to an injection compression molding method and an injection compression molding machine for molding products by molding thermoplastic resin being injected and compressed.
2. Description of Related Art
Injection compression molding method is known as a molding method for obtaining lenses from thermoplastic resin as disclosed in Japanese Patent Publication Laid-Open No. Hei 2-26723 and Japanese Utility Model No. Hei 6-9826. Incidentally, spectacle lenses having lens power ranging from +4.00 to xe2x88x928.00 D(D: diopter) are preferably used for the molding method.
The injection compression molding method is a molding method for correcting shrinkage of the molten resin to obtain uniform and highly accurate configuration, in which molds are closed while retaining a predetermined compression margin in a lens forming cavity and, subsequently to injecting the molten resin to fill the lens forming cavity, the compression margin is compressed to obtain lenses.
Incidentally, when curvature of front side and rear side of the lens differ, thickness difference is caused to the thickness of the lens. As lenses having the thickness difference, there are two kinds of lenses, i.e. a plus lens having thicker central portion than peripheral portion thereof and a minus lens having thinner central portion than peripheral portion thereof.
The lens thickness differs according to a design of an optical lens. For instance, an aspheric lens having refractivity of approximate 1.50 and lens power of +2.00 D has approximate central thickness of 4.2 mm and approximate peripheral thickness of 1.0 mm. When the lens power is xe2x88x922.00 D, the central thickness is designed to be approximately 1.4 mm and the peripheral thickness is designed to be approximately 4.8 mm. When the lens power is xe2x88x924.00 D, the central thickness is designed to be approximately 1.44 mm and the peripheral thickness is designed to be approximately 7.9 mm.
In injection-compression-molding such lenses having the thickness difference, following disadvantage occurs according to difference in configuration characteristics.
Since the peripheral portion of the plus lens is thinner than the central portion thereof, a deformation containing molding shrinkage called xe2x80x9chi-kexe2x80x9d (surface sink) is likely to be caused around the gate in molding plus lenses, thereby making it difficult to keep configuration accuracy of the peripheral portion.
Since the central portion of the minus lens is thinner than the peripheral portion thereof, central portion of the cavity has greater flow resistance. Therefore, the molten resin injected into the cavity flows to branch into the peripheral portion and around into the central portion on account of difficulty of flowing the central portion of the cavity, thereby often causing weld mark in the central portion.
Accordingly, conventional molding die for the plus lens has a small gate opening configuration in order to restrain the development of the xe2x80x9chi-kexe2x80x9d (surface sink).
On the other hand, since the weld mark tends to be generated when the molten resin is not smoothly flown into the cavity, conventional molding die for the minus lens has a relatively large gate opening configuration in order to ensure the fluidity of the molten resin.
However, since there is delicate difference in the gate opening configuration according to lens thickness difference, numerous molding dies having different gate opening configuration have to be prepared, thereby increasing financial burden.
In view of the above, the Applicant of the present application has proposed an injection molding die for molding lens, in which the gate portion is arranged to be exchangeable (Japanese Patent Application No. Hei 8-6407).
The gate portion of the molding die is arranged to be exchangeable. Furthermore, a plurality type of gate top member for defining the gate opening configuration is prepared in advance. A gate top member having gate opening configuration corresponding to the lens configuration to be molded is selected from the gate top members and is exchangeably attached to the gate portion of the molding die.
However, even when the gate top member is selected in accordance with configuration property of the lens to be molded, the gate top member has to be exchanged in accordance with the lens configuration to cause much trouble, and the productivity can not be greatly improved.
Further, it is usually preferable that the gate is arranged to have small resistance in filling the cavity with the molten resin and the molten resin is preferably shut in the cavity after the cavity is packed with the molten resin. Accordingly, the configuration and the size of the gate has to be determined by a compromise between the conflicting demands.
Accordingly, as mentioned above, even when the gate top members having the gate opening configuration in accordance with the lens configuration are prepared in advance, since the gate opening configuration has to be determined by an appropriate compromise of the contradictory demands, the contradictory demands can not be satisfied simultaneously. Further, in order to satisfy the demands as best as possible, the gate opening configuration gets too complicated, thereby increasing production cost.
The object of the present invention is to provide an injection compression molding method and injection compression molding machine for solving the above-mentioned conventional problem to improve the productivity and for obtaining molding products having high configuration accuracy and high quality.
The present invention is made by taking notice of the fact that both of the above two demands for the gate (fewer or lower resistance in filling the molten resin into the cavity and shutting in the molten resin after the molten resin is packed in the cavity) can be met utilizing time lag between the respective demands, in which the opening of the gate is closed after injecting and filling the molten resin into the cavity.
A injection compression molding method according to the present invention includes the following steps: providing an openable/closable molding die having at least one cavity including a pair of relatively movable cavity forming member and a resin flow channel in communication with the cavity through a gate; closing the molding die; setting a size of the cavity by relatively moving the pair of cavity forming members to form a predetermined compression margin inside the cavity; injecting molten resin to fill the cavity; and at least after a time point prior to completion of injection of the molten resin, reducing the size of the cavity by relatively moving the pair of cavity forming members to compress the molten resin. The present invention is characterized in that an opening of the gate is shut after completion of injecting the molten resin; and the gate shut operation is conducted in synchronization with the relative movement of the pair of cavity forming member in reducing the size of the cavity.
In the above, the shutting timing of the opening of the gate is set after completion of injection of the molten resin, more preferably, immediately after completion of injection of the molten resin, and the shutting action of the opening of the gate may be initiated before completion of injecting the molten resin.
When the opening of the gate is closed, a clearance enough to prevent the molten resin injected and filled in the cavity from flowing back to the resin channel from the gate during cooling process, preferably a clearance of less than or the same as 2 mm may be secured. Or alternatively, the opening of the gate may be completely closed.
Further, the number of the cavity may either be single or plural. The type of the cavity is not limited, but a lens-forming cavity having meniscus shape, which requires high accuracy and high quality, is preferably employed.
According to the injection compression molding method, since the size of the cavity is initially set; the molten resin is injected and filled into the cavity; the opening of the gate is closed in synchronization with the relative movement of the cavity forming member in reducing the cavity volume to compress the molten resin; and the opening of the gate is closed after completion of the injection of the molten resin etc., the molten resin can be prevented from flowing back to the resin flow channel from the gate in the cooling process. Accordingly, a product having high accuracy and high quality can be molded restraining development of internal strain.
Therefore, since the opening configuration of the gate opening in flowing the molten resin into the cavity can be determined considering only fluidity of the molten resin, the gate opening configuration and the size are not required to be changed in accordance with the configuration of the products to be molded. Accordingly, for instance, the conventional process of changing gate top member having gate opening configuration corresponding to the configuration of the lens is not required, thereby improving the productivity.
In the above injection compression molding method, the shutting operation of the gate may be conducted in direct link with the relative movement of the pair of the cavity forming member in reducing the cavity, or alternatively, by a second driving means independent of a first driving means for relatively moving the pair of cavity forming member in reducing the size of the cavity.
According to the above, the opening of the gate can be shut after completion of injection of the molten resin. For instance, when the cavity volume is reduced after completion of injecting the molten resin, the opening of the gate can be closed after initiation of reducing the cavity volume. When the cavity volume is reduced before completion of injecting the molten resin, the opening of the gate can be initiated to be closed after initiation of reducing the cavity volume and the opening of the gate can be closed after the molten resin is completely injected.
Accordingly, when the molten resin inside the cavity is gradually compressed by the relative movement of the pair of cavity forming member to increase the pressure applied to the molten resin, since the opening of the gate is closed, the resin can be securely prevented from flowing back to the resin channel from the gate.
Another injection compression molding method includes following steps: providing an openable/closable molding die having at least one cavity including a pair of relatively movable cavity forming member and a resin flow channel in communication with the cavity through a gate; closing the molding die; setting a size of the cavity by relatively moving the pair of cavity forming member to form a predetermined compression margin inside the cavity; injecting molten resin to fill the cavity; and at least after a time point prior to completion of injection of the molten resin, reducing the size of the cavity by relatively moving the pair of cavity forming member to compress the molten resin. The injection compression molding method is characterized in that the predetermined margin inside the cavity and an opening space of the gate is set in accordance with configuration property of the lens to be molded, that an opening of the gate is shut after completion of injecting the molten resin, and that the gate shut operation is conducted in synchronization with the relative movement of the pair of cavity forming member in reducing the size of the cavity.
According to the injection compression molding method, even when the predetermined compression margin inside the cavity and the opening space of the gate is set in accordance with configuration property of the lens, e.g. plus lens and minus lens, or even when the opening space of the gate is opened when the predetermined margin inside the cavity is set as, for instance, approximately 0.1 mm when the lens power is +2.00 D, set as approximately 7 mm when the lens power is xe2x88x922.00 D, and set as approximately 12 mm when the lens power is xe2x88x924.00 D, the molten resin can be prevented from flowing back to the resin channel from the gate, since the opening of the gate is closed after completion of injecting the molten resin. Further, above setting is preferably changed in accordance with material property such as fluidity of the resin of the lens. Accordingly, the predetermined compression margin inside the cavity and the opening space of the gate can be selectively set in accordance with the configuration of the lens without considering backflow of the molten resin from the gate to the resin flow channel.
For instance, in molding the plus lens, the configuration of the opening of the gate can be set small in order to restrain the xe2x80x9chi-kexe2x80x9d (surface sink) as least as possible. In molding minus lens, the predetermined compression margin in the cavity and the opening space of the gate can be set large. Incidentally, when the predetermined margin in the cavity and the opening space of the gate is set large in molding minus lens, the resin fluidity at the gate can be secured since the molten resin flows into the cavity through the large-set gate. Further, since the molten resin flowing into the cavity can be flown into peripheral portion thereof through the central portion of the cavity without branching by virtue of the greatly-set space in the cavity, the development of the weld mark at the central portion can be prevented.
In the above injection compression molding method, the opening space of the gate may be driven and controlled by a second driving means independent of a first driving means for setting and reducing the size of the cavity, and the shutting operation of the gate may be conducted by the second driving means.
Accordingly, since the opening space of the gate can be selectively set as a desired size independently of the cavity size setting process and the opening of the gate can be shut after completion of injecting the molten resin, the molten resin can be securely prevented from flowing back to the resin flow channel from the gate by virtue of the closed opening of the gate even when the pressure applied to the molten resin increased.
In the above, the configuration of the lens may be defined by a grouped unit of a plurality of approximate lens power.
Accordingly, the predetermined compression margin in the cavity and the opening space of the gate can be selectively set for each unit of the lens power. In other words, since the molding condition can be changed in accordance with the lens power, the lens can be molded with higher accuracy.
An injection compression molding machine according to the present invention has an openable/closable molding die having at least one cavity including a pair of relatively movable cavity forming member and a resin flow channel in communication with the cavity through a gate, a cavity setting means for closing the molding die and for relatively moving the pair of cavity forming member to set the size of the cavity so that a predetermined compression margin is formed inside the cavity, an injection means for injecting a molten resin to fill the cavity set by the cavity setting means, a cavity reduction means for relatively moving the pair of the cavity forming member to reduce the size of the cavity at least after a time point prior to completion of injecting the molten resin, and a gate control means for shutting an opening of the gate after completion of injecting the molten resin in synchronization of the relative movement of the pair of cavity forming member in reducing the size of the cavity.
According to the injection compression molding machine, the molding die is closed and the size of the cavity is set by relatively moving a pair of cavity forming member. Subsequently, after the molten resin is injected and filled in the cavity, the molten resin is compressed by relatively moving the pair of cavity forming member at least after a time point prior to completion of injecting the molten resin. In the above, since the opening of the gate is closed after completion of injecting the molten resin, the molten resin can be prevented from flowing back to the resin flow channel from the gate. Accordingly, a product having high accuracy and high quality can be molded by restraining development of internal strain. Further, since the conventional exchanging step of the gate top member having a gate opening configuration corresponding to the lens configuration is not required, the productivity can be improved.
In the above, though any arrangement is possible for the gate control means as long as the opening of the gate can be shut after completion of injecting the molten resin, the gate control means preferably includes a gate shut member protrudable into the gate, the gate shut member being fixed to a movable-side cavity forming member of the pair of the cavity forming member.
In the above, the gate shut member can be easily synchronized with the relative movement of the cavity forming member by fixing to a movable-side cavity forming member of the pair of cavity forming member. Accordingly, since the gate shut member protrudes into and retreats from the gate by the relative movement of the cavity forming member, the gate opening configuration can be determined when the pair of cavity forming member relatively moves by the cavity setting means. Therefore, the opening configuration of the gate during the flowing step of the molten resin into the cavity can be selectively set by determining the protruding amount of the gate shut member protruding into the gate.
Alternatively, the gate control means may have a gate shut member protrudable into the gate, the gate shut member being provided to a movable-side cavity forming member of the pair of the cavity forming member movably in a movable direction of the movable-side cavity forming member, and a driving means for moving the gate shut member independently of the movable-side cavity forming member.
Accordingly, as mentioned above, since the opening of the gate can be shut after completion of injecting the molten resin, the molten resin can be securely prevented from flowing back into the resin flow channel from the gate by virtue of the closed opening of the gate even when the pressure applied to the molten resin increased by the relative movement of the pair of the cavity forming member to compress the molten resin.
Further, the pair of cavity forming member may preferably be optical inserts for molding spectacle lens disposed in the molding die independently of the gate, the optical insert being exchangeable in accordance with a power of the lens to be molded.
Accordingly, the spectacle-lens forming optical insert can be exchanged in accordance with the lens power of the spectacle lens, and, furthermore, since the gate and the gate shut member can be continuously used, the gate shut member is not required to be exchanged for each time the insert is exchanged.